Effect of Tempering on the Ductile-to-Brittle Transitional Behavior of Ni-Cr-Mo Low-Alloy Steel

Background: About 10 years ago, super-high energy Charpy specimens at the National Institute of Standards and Technology were removed from inventory due to unacceptable variability in absorbed energy, leading to the advent of new methods and materials to reduce the variability and maintain the prescribed energy levels. Objective: In this paper, we investigated the ductile-to-brittle transitional behavior of Ni-Cr-Mo low-alloy steel by testing Charpy specimens with side-grooves as a function of the final temper temperature to define the processing conditions for these super-high energy levels. Methods: For each temper, absorbed energy and force-displacement data were measured as a function of test temperature; the former was used to assess transition temperature and upper-shelf energy and the latter was used to estimate shear fracture appearance (SFA). Results: From the upper-shelf energy results, it was found that two of the temper conditions yielded energies in the super-high energy range and that side-grooves reduced the variability of the energy by preventing the formation of shear lips. From the SFA data, it was shown that the instrumented striker data and fractography were in excellent agreement, with the minor discrepancies attributed to difficulties with transitional fracture surfaces in the fractography and multiple crack arrest points in the instrumented striker data. Conclusions: In all, the data provided clear evidence that Ni-Cr-Mo low-alloy steel is a good solution for super-high energy Charpy indirect verification specimens.